1. Field of the Invention
The present invention relates to nanophosphor and synthesis method thereof, and particularly to fluoride-based dual-mode emission nanophosphor which can be excited by infrared (IR) rays and ultraviolet (UV) rays to emit visible light and can have magnetic property, and synthesis method thereof. The nanophosphor can be used as contrast agent for magnetic resonance imaging or fluorescent imaging agent and applied to counterfeit prevention code and solar cell.
2. Background of the Invention
Different from semiconductor nanocrystals, light emitting nanoparticles doped with lanthanide elements have a characteristic that the peak positions of emission spectrum does not change even though its particle diameter is altered.
This is because light emission of the nanoparticles doped with lanthanide elements, namely nanophosphor, is generated by 4f electron transition of the lanthanide element such as 4f-4f or 4f-5d transition. Therefore, when using light emitting nanoparticles doped with lanthanide elements, there is an advantage that even though the particle diameter is adjusted diversely, it is possible to keep desired emission wavelength.
The light emission of nanophosphors occurs mostly when they are excited by ultraviolet rays or visible rays. However according to types of element to be doped with, it is possible to synthesize nanophosphor emitting visible light after excitation by IR rays.
Fluoride-based nanophosphor co-doped with Ytterbium (Yb) and Erbium (Er) shows emission peaks in green and red spectrum range after excitation by IR rays. In this case, as IR ray having low energy is converted to visible light having high energy, this is referred as up-conversion emission, distinguished from down-conversion emission to emit visible light via energy loss process after excitation by UV rays [Chem. Rev. vol. 104, 139-174 (2004)].
When using this up-conversion emission in applying it to fluorescent imaging agent, the penetrating depth of excited light into a cell becomes deeper and clear signal can be obtained as the excited light, IR rays, induces no self-light emission.
Especially, NaYF4 is known as the most efficient host compound displaying up-conversion [Chem. Mater. vol. 16, 1244-1251 (2004)]. NaYF4:Yb,Er nanophosphor shows up-conversion emission via absorption of near IR rays by ytterbium and energy transfer to erbium, so it shows very high efficiency than up-conversion emission by two photon absorption. Due to the above-mentioned difference of light emitting implement, there is no need to high price pulse type laser device and it is possible to display up-conversion emission with low price diode laser.
However, NaYF4 shows polymorphism of cubic alpha (α) phase and hexagonal beta (β) phase, which has a feature that the beta (β) phase shows better emission than the alpha (α) phase. In this case, as extremely high synthesis temperature is needed to obtain the beta (β) phase, which makes diameter of particles to be synthesized larger and induces clustering among particles, there is a demerit that it is not suitable for biological applications [J.Phys. Chem. C vol. 111, 13730-13739 (2007)]. In case that the particle diameter is made smaller by adjusting synthesis condition to overcome this problem, another problem that surface defect increases and the emission intensity drops rapidly occurs.
Therefore, there is an urgent requirement to develop nanoparticles with smaller diameter as well as display of strong up-conversion emission or develop multifunctional nanoparticles able to produce another signals as well as up-conversion emission.
Furthermore, if it was possible to obtain various signals such as down-conversion emission and magnetic properties other than up-conversion emission, the properties as contrast agent could be improved largely.